The present invention relates in general to exposing patterns with radiant energy passing through masks and more particularly concerns novel apparatus and techniques for exposing patterns of spatial period p/n using near-field diffraction from masks of spatial period p with n being an integer greater than 1. The invention may be used with visible, UV or X-ray radiation (as well as electrons or ions) to produce gratings of exceptionally fine spatial period. A list of numbered references is appended.
Periodic and quasi-periodic structures of submicrometer spatial period are of fundamental importance in a number of areas such as guided wave optics as described in (1), distributive-feedback and distributed-Bragg-reflector lasers as described in (2), graphoepitaxy as described in (3) and (4) and planar superlattice electronic devices as described in (5). In addition high-quality periodic structures are essential elements in optics and spectroscopy at all wavelengths, and serve as calibration standards for scanning electron beam systems as described in (6). For spatial periods from about 2 .mu.m to of the order of 0.2 .mu.m, holographic lithography has been the preferred method of exposing periodic and quasi-periodic patterns, either directly on substrates of interest or on masks which are subsequently replicated photolithographically or with soft X-rays as described in (8). Holography in high-refractive-index media has been used to expose gratings of 110 nm period as described in (9), but the technique is cumbersome and difficult to extend for periods much below about 70 nm. Sources with wavelengths shorter than the He:Cd laser (325 nm) generally have limited coherence length and poor mode quality, or require complex apparatus for up-conversion as described in (10).
Accordingly, it is an important object of the invention to provide improved apparatus and techniques for exposing spatial periods, typically below 10 .mu.m, useful in one or more of the applications described above and also in very-far UV and X-ray optics.
It is another object of the invention to achieve the preceding object with apparatus and techniques that are relatively free from complexity.
It is a further object of the invention to achieve one or more of the preceding objects, especially in conjunction with soft X-ray lithography.
It is an object of the invention to employ near-field diffraction from periodic and quasi-periodic "parent" masks to produce intensity patterns with spatial periods finer than the parent mask, a phenomenon previously observed with visible light and described in (11).